Machining steels



Patented Oct. 30, 1945 2,388,215 MACHINING STEELS Donald William Murphy, Bethlehem, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing.

Original application August 27,

1941, Serial No. 408,493. Divided and this application August 6, 1945, Serial No. 609,325

1 Claim.

My invention relates to machining steels. My invention comprises free-machining or allied steels havin a small percentage of boron. It also comprises such steels containing other elements to give machinability effects, such as sulphur, phosphorus or nitrogen.

I have discovered that boron in very sma amounts in steel contributes markedly to its machinability. As is well known, there are definite limits to the life of a cutting tool when used to machine steels, and definite limits to the speed at which the tool may be used. As the speed is increased, the life of the tool is shortened. I have found that when 0.01% or even lesser amounts of lggmn are added to steels the life of the tool used thereon is greatly increased for a given speed, and conversely, for a particular tool life, the speed may be greatly increased.

I employ boron in amounts from 0.004% to 0.02%, usually preferring to use in the neighborhood of 0.01%.

My invention is particularly efficacious when both boron and sulphur are used in steels. Such steels possess machinability properties considerably in excess of those containing sulphur alone, and moreover, the presence of the boron in such steels overcomes to a considerable degree certain disadvantages of sulphur in steel.

Sulphur, because of certain deleterious effects, is normally kept low in steels, the sulphur content ordinarily running from about 0.02% to about 0.04%. However, it is well known practice to make free-machining steels by increasing the sulphur content so that the sulphur in open hearth steels runs from about 0.07% to 0.20% and, in the case of Bessemer steels, from about 0.10% to 0.30%. These higher contents of sulphur render such steels capable of being machined to give longer tool life (conversely, greater speeds with the same tool life), and the machined surface is of better character. The addition of these higher amounts of sulphur, however, has the effect of producing a steel in which the sulphur tends to be more or less segregated. Furthermore, such steels are more or less hot short depending upon the amount of sulphur present.

By including boron in such steels of higher sulphur content, I greatly increase the machinability characteristics and I overcome to a considerable degree some of the disadvantages of the higher sulphur contents.

For example, a steel containing 0.15% sulphur possessed a cutting speed of 162 surface feet per minute for 100 minutes tool life. A steel containing the same sulphur content, namely, 0.15% sulphur, but also containing 0.01% boron, possessed a cutting speed of 219 surface feet per minute for minutes tool life, an increase of about 35%.

In addition to increasing the machinability of sulphur containing steels, the boron greatly reduces the tendency of the sulphur to segregate, thereby giving a much more uniform character to the steel.

I obtain substantial effects on sulphur-bearing steels as regards machinability and uniformity of product, by the use of various amounts of boron. The boron content may be varied from 0.004% to 0.015%, using sulphur in amounts from 0.06% to 0.3%.

As indicated above, sulphur renders steels more or less hot short, this characteristic increasing with increasin amounts of sulphur. Steels are frequently needed possessing a high degree of machinability and, at the same time, having a low order of hot shortness. But to obtain the necessary machinability by means of sulphur it frequently happens that such large amounts of this element are required as to be incompatible with the demand for a relatively low order of hot shortness. Boron, while increasing the cutting speed, has relatively little effect as regards hot shortness in amounts of the order of 0.01% and less. I take advantage of this property of boron by reducing the sulphur content the necessary amount to lessen the hot shortness to the point of toleration, and adding boron to give the necessary cutting speed.

Examples of analyses of sulphur-boron steels which I have found to be eifective are as follows:

C Mn P S Si Percent Percent Percent Percent Percent (1) 22 .63 014 05 (2) ll 73 077 304 009 (3) l0 67 08 19 01 (4) 20 85 08 13 .05 (5) 20 85 015 l3 05 (6) 20 85 015 l3 05 the boron content of these analyses varying between 0.004% and 0.02%.

I have also found that boron and nitrogen cooperate very effectively in steels having freecutting characteristics. Nitrogen by itself, when used in sufficient amount, gives good surface to machined steels and also causes the steel to chip effectively during the cutting operation. Nitrogen by itself, however, apparently somewhat decreases cutting speeds, as nitrogen increases Mn S N B Percent Percent Percent Percent Percent (1) 18 60 029 01 007 (2) l5 80 041 009 007 I have found suitable combinations of sulfur, nitrogen and boron to give excellent efiects, giving steels capable of being machined at high speed and producing products with excellent finish. As specific examples of these combinations of sulphur, nitrogen and boron, I submit the following analyses:

0 Mn S N B Percent Percent Percent Percent Percent (l) 18 78 148 009 01 (2) 1.03 216 011 007 I have also found that effective steels for machining purposes may be produced by using phosphorus over the normal amount in conjunction with boron. In steels of this sort I prefer to use boron between 0.004% and 0.02%, and phosphorus between 0.035% and 0.12%. As a specific example I give the following analysis:

0 Mn S P B Percent Percent Percent Percent Percent .18 .80 .04 .081 .01

In my steels the carbon content will vary from low carbon up to 0.50%, ordinarily running between 0.10% and 0.50%.

Normally, with my steels, the manganese will run from 0.4% to 1.60%, although higher or less manganese contents may be used. Ordinarily it is well to have a substantial content of manganese as this element tends to overcome hot shortness. I have employed manganese as high as 2.5% without any disadvantageous counteracting effects upon the boron.

Boron may be used to contribute advantageous effects, recited above, in steels containing up to 16% chromium and I have found that it may be used in steels containing varying amounts of nickel, say up to 3%, or possibly higher. I have found that the boron addition contributes its useful efiects in nitrogen-chromium combinations including the austenitic varieties of these steels.

In preparing my steels, containing boron, the boron is added in an alloy form either to the steel in the ladle or to the mold. I usually employ ferro-boron containing 10-20% of elemental boron. I have found that adding the ferro-boron to the mold is somewhat more economical than adding it to the ladle. When adding to the ladle it is usually necessary to employ somewhat more ferro-boron than when adding it to the mold, to get the same boron content in the finished product.

This present application is a division of my application Serial No. 408,493, filed in the Patent Ofiice August 27, 1941.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

A free-machining steel containing carbon from 0.10% to 0.50%; sulphur from 0.06% to 0.30%; boron from 0.004% to 0.01%; nitrogen from 0.004% to 0.018% and the balance substantially all iron.

DONALD WILLIAM MURPHY. 

